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on Monday September 23, 2013 @10:57PM
from the ten-meters-tall dept.

carmendrahl writes "Eukaryotic cells, which are defined by having a nucleus, rarely grow larger than 10 micrometers in diameter. Scientists know a few reasons why this is so. A new study suggests another reason — gravity. Studying egg cells from the frog Xenopus laevis, which reach as big as 1 mm across and are common research tools, Princeton researchers Marina Feric and Clifford Brangwynne noticed that the insides of the eggs' nuclei settled to the bottom when they disabled a mesh made from the cytoskeleton protein actin. They think the frog eggs evolved the mesh to counteract gravity, which according to their calculations becomes significant if cells get bigger than 10 micrometers in diameter."

What's your point? Do you know how fast a station of a particular size would have to spin to generate a reasonable amount of artificial gravity? Do you know how said motion would affect the vestibular system of the average human?

The first part of your question is easy enough to calculate: a=omega^2*r => omega=sqrt(a/r)

So for earth-normal gravity (although it's likely that Mars- or moon-gravity would suffice), 100m radius ring (or dumbbell), you'd rotate at 0.313 radians per second [google.com] or about 1 revolution every 20 seconds. Can the human body handle that without puking? No idea.

I wouldn't call it easy. Sure, in a relatively non-mobile space station that was reasonably small it would be a trivial problem. However, changing the direction of a spinning object at high speeds is no simple task, and at a certain size the station would pull itself apart unless it was made of some sort of super strong exotic metal.

Plus, a structure like that would be hard to maintain over a long period of time since a self sustaining micro-ecosystem would need a body of water of some sort, and any leaks i

at a certain size the station would pull itself apart unless it was made of some sort of super strong exotic metal.

Attach space station to counterweight with a bunch of long tethers.Spin to taste.We've got cables holding up rather heavy stuff down on Earth.No need to spin at high speeds, no need for big space stations.

I wouldn't call it easy. Sure, in a relatively non-mobile space station that was reasonably small it would be a trivial problem. However, changing the direction of a spinning object at high speeds is no simple task

For space stations we don't want to change direction drastically, we only want to make minor adjustments. Along the spinning axis is trivial, in any other direction the thrust could be synchronized with the rotation. Depending on how advanced you want it to be you may have to adjust the rotation again after adjusting the direction.

and at a certain size the station would pull itself apart unless it was made of some sort of super strong exotic metal.

The force we are talking about is one earth gravity. That is, the force is no larger than what hanging bridges or building floors already have to deal with. The super strong exoti

A possible solution would be to have an entrypoint at the center of the rotation and have an elevator down to the station.

However if you dock material in the center and move it to the main part of the station you will rob angular momentum from the station as a whole. Similarly moving material to the center to move it will add angular momentum to the station as a whole. Also changes in the mass in the main part of the station will move the center of mass and hence the center of rotation.

Didn't you see what happened to the Tardigrades that escaped from the ISS? They have formed a group called the Spacer's Guild. They are rad hard, can live in vacuum and can be their own space ship. Now that I think of it, that might be an interesting space station, very organic punk.

There was some concern about the effects of weightlessness. I mean, it's a constant on earth, it makes sense that animals would have used it for some developmental signal. My thesis adviser worked on a collaboration between soviet scientists and US scientists in the 80s, testing if there were any effects on incubating and hatching eggs in microgravity.

Evidently the most interesting result to come out of that study was that quail embryos preserved in vodka are not easy to section for microscopy. Parafo

It isn't referring to generic cellular actin, it's referring to the nuclear actin which the larger cells had a much higher concentration of.
Its obvious even if you just read the abstract of the actual article, just not the shitty summary on C&EN.

This sounds like a perfect experiment for ISS. They mainly do biological experiments (it's not really a good platform for anything else), and this could be a neat result. CASIS (the ISS science institute) is always looking for new experiments and experimenters for the station.

Scientists have been doing stem cell (mostly plant stem cells, but also some mammalian etc.) growth experiments on the ISS for some years (IIRC six flights so far). Results are interesting. Among other things, perhaps the two most interesting results have been as follows.

In microgravity, cell growth is not limited to 2D. For example, that $250,000 hamburger was made by growing hundreds or thousands of one-cell-thick strips on petri dishes. In space, that is no longer the case. So stem cells can be grown one or two orders of magnitude faster, limited only by the need to get nutrients delivered to each cell and wastes removed.

Some mammalian cells that are very difficult or so far impossible to grow down here on Earth have been shown to grow pretty well up there in microgravity, including some human tissue types.

While some form of life on Earth has encountered and adapted to almost every other environmental condition (temperature, light, pH, etc.), so far as we know no living systems have ever had to deal with microgravity. So when grown in space, the cells basically 'freak out', not knowing what to do, and apparently try turning all of their genes to see what works. This seems to make them more amenable to influence by the environment, such as by adjusting temperature outside the norm for the species. Zero Gravity Solutions [zerogsi.com], a biotech company, is preparing further experiments on the ISS to explore this and related questions. (disclosure: I have a small investment in ZeroGSI.)

I've just realized (though I'm sure someone smarter than me might have done this before) that the fact that cells are more or less spherical means that they evolved microgravity or while buoyant in a liquid. So it's either the aliens in an ancient station in orbit or earth's oceans.

By osmosis. Imagine inflating a water balloon, if you pump a heavier fluid like water into it, even in the presence of gravity it will take on a spherical shape. Of course it will be *slightly* deformed, but that's all up to the ratio of weight:surface strength.

The only cells that are spherical are floating. And not all floating cells are spherical (yeast, for example).
The cells of most multicellular organisms take on a shape by adhering to each other to to an extracellular matrix, and they generate internal tension by pulling on the adhesions. When you disaggregate the tissue, the individual cells still try to maintain that tension, but with nothing to pull against tend to pull the cell into a little ball.

And if you believe in panspermia, could this mean an investigation of ancient life cell sizes could give clues as to the specific gravity that designed that cell, hinting at the gravity environment they may have originated from?

Gromia sphaerica is a large spherical testate amoeba, a single-celled organism classed among the protists and is the largest in the genus Gromia. It was discovered in 2000, along the Oman margin of the Arabian sea, at depths from 1163 to 1194 meters (3816 to 3917 feet). Specimens range in size from 4.7 to 38 millimeters (0.2 to 1.5 inches) in diameter.

I was under the impression that surface area played a significant role in this as well, which probably couldn't be easily discounted. The surface area of a sphere is 4 pi times the radius squared while the volume is 4/3 pi times the radius cubed. So, the greater the radius, the greater the ratio of volume to surface area. This usually doesn't scale well, as it means there is more mass to support and less means of getting the input and output needed to support it. I'm not saying that gravity doesn't contribute as well, but that's a fairly difficult barrier as well.

I suspect in general the distance from the nucleus to every place else in the cell matters. That's why settling to the bottom is bad.
But volume compared to surface area is a major limiting factor for any biological thing.
And even for the fusion processes of suns. Once the hydrogen fire's pressure eases, the star collapses. Quickly.
It's also why it takes a lot longer to make 4 3d copies on a 3d printer than it does to copy 4 sheets of paper.

Does this mean there is also a "goldilocks size" to a planet for advanced life, even any life to evolve? Can it be at the correct distance from its star, have water, and still be essentially dead? Can a planet look like the Earth but because it is five times the size, be sterile?

Good point. Just to nitpick: A planet's surface gravity is function not simply of it's size (volume), but its mass. That means density must also be taken into account. You could conceivably have a planet smaller than Earth with >1g gravity if it was made of more dense stuff than Earth.

Therefore your question would be about a "Goldilocks mass" rather than a "goldilocks size".

I suspect that life could still evolve under higher gravity conditions, but it would have to develop with smaller cell sizes/ differ

Couldn't a cell be neutrally buoyant in water negating the effects of gravity? I thought it was something like surface tension keeping these things small, it would also make sense that this mesh would counteract surface tension.